Year in Review: Sleep clears the cluttered brain

Gunk between cells is cleansed during slumber

SLEEP CLEANER Tracer dyes (green, red) show where cerebrospinal fluid floods into the brain when a mouse is asleep. The fluid helps cleanse the brain of waste products that can build up and damage brain cells. Fluid flow nearly stops when the mice are awake.

Sleep showers away cellular grime that builds up while the brain is awake — just the sort of process that could have made sleep a biological imperative, scientists reported in October (SN: 11/16/13, p. 7).

People have long puzzled over the evolutionary pressures that led animals to need sleep even though it leaves them vulnerable to predators and other dangers. Rinsing off the brain and disposing of waste proteins and other gunk might help explain why sleep evolved.

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